Conventionally, a molding method that combines a primary molding by extruding and a secondary molding by blowing (or vacuuming) has been used for manufacturing, for example, a sandwich panel made of resin.
With such molding method, a blow (or vacuum) molding is performed directly using an extruded resin in a molten state. This ensures molding the sandwich panel without posing a technical problem, such as a non-uniformity of heating, caused by reheating a resin that is once molded.
Especially, performing the blow (or vacuum) molding after causing the extruded resin in the molten state to directly droop downward and mold-clamping the resin extending in a vertical direction ensures sending out the resin from an extrusion die in an untouched state without requiring a support of the resin in the molten state until the mold clamping in the secondary molding compared with a case where, for example, the resin is extruded in a lateral direction.
However, in these preceding molding techniques, the extruded resin in the molten state directly droops downward before the secondary molding. In view of this, a draw-down or a neck-in that is generated in a sheet in a molten state poses a technical problem that is a thickness of the sheet before molding by a mold becomes non-uniform in an extruding direction or a width direction of the sheet. Hence, the applicant proposes a novel molding technique as follows in Japanese Patent No. 4902789. Here, the draw-down is a phenomenon in which the sheet in the molten state is stretched by the sheet's own weight with a lapse of time and the upper the portion of the sheet, the thinner it gets. The neck-in is a phenomenon in which the sheet contracts in the width direction and a sheet width decreases due to the draw-down.
That is, this molding method for a resin molded article includes a step of extruding a thermoplastic resin downward into a sheet shape in a molten state so as to droop downward, a step of sandwiching the sheet-shaped resin in the molten state excluded downward by a pair of rollers and sending out the sheet-shaped resin downward by a rotational driving of the rollers, a step of disposing the sheet-shaped resin in the molten state sent out downward in a side portion of a mold disposed below the pair of rollers, and a step of molding the sheet-shaped resin into a shape conforming to a mold shape by depressurizing a sealed space formed between the sheet-shaped resin in the molten state and the mold and/or pressurizing the sheet-shaped resin toward the mold. Especially, the pair of rollers are brought relatively close to one another after a lowest portion of the sheet-shaped resin in the molten state extruded downward passes between the pair of rollers. With this, the pair of rollers sandwich the sheet-shaped resin and the rotational drive of the roller sends out the sheet-shaped resin downward at a speed equal to or more than a predetermined extrusion speed.
With such molding method for the resin molded article, as the sheet-shaped resin is sent out downward by the pair of rollers, a length of the sheet-shaped resin drooping in the vertical direction becomes long. Due to this, the upper the portion of the drooping sheet-shaped resin, the thinner it gets by the sheet-shaped resin's own weight (the draw-down or the neck-in). On the other hand, adjusting a rotation speed of the roller so as to make a sending out speed by the pair of rollers become equal to or more than the extrusion speed causes the sheet-shaped resin to be pulled downward by the pair of rollers, thus the sheet-shaped resin is stretched and thinned.
At this time, the rotation speed of the rollers is decreased with a lapse of time and the sending out speed is adjusted so as to come close to the extrusion speed of the sheet made of the thermoplastic resin. In view of this, the upper the portion of the sheet-shaped resin, the more the downward tensile force by the pair of rollers decreases. Hence, stretching and thinning in association with such tensile force is relatively reduced and thinning in association with the draw-down or the neck-in is cancelled out, thereby effectively preventing the draw-down or the neck-in. As a result, a uniform thickness can be formed in the extruding direction.
Next, the sheet-shaped resin formed so as to have the uniform thickness in the extruding direction is disposed in the side portion of the mold disposed below the pair of rollers. Depressurizing the sealed space formed between the sheet-shaped resin and the mold and/or pressurizing the sheet-shaped resin toward the mold molds the sheet-shaped resin into the shape conforming to the mold shape. This ensures molding a resin molded article having a desired thickness in the extruding direction without adversely affecting a forming in the secondary molding.
However, in this molding method, the applicant has found points that cause new technical problems described below that are caused by adjusting the thickness of the sheet-shaped resin in the molten state exclusively by the pair of rollers.
First, suppose that the thinning of the resin sheet in the molten state is performed only by sending out the resin sheet in the molten state by the pair of rollers. In this case, a progress of sending out and a restraining of the draw-down generated by the resin sheet's own weight by the adjustment of the rotation speed of the pair of rollers, that is, the sending out speed, have limitations. The resin sheet sent out from the pair of rollers is no longer adjusted afterwards, and can only be thinned in a range considering that the resin sheet is pulled and stretched by its own weight. That is, depending on a length (weight) of the resin sheet in the molten state that droops from the pair of rollers, a tear of the resin sheet in the molten state possibly occurs. In view of this, the thickness of the resin sheet had to be determined in a range considering the draw-down. Furthermore, in order to thin the sheet-shaped resin in the molten state, accelerating the rotation speed of the pair of rollers to increase the sending out speed of the sheet-shaped resin can restrain the draw-down along the sending out direction of the sheet-shaped resin, however, the neck-in is generated in the width direction that is a direction perpendicular to the sending out direction of the sheet-shaped resin. In view of this, achieving further thinning becomes difficult or the secondary molding is adversely affected.
Second, the sheet-shaped resin in the molten state continuously sent out by the pair of rollers contacting the rollers at a normal temperature cools down a surface of the sheet-shaped resin and formability in the secondary molding is adversely affected. More specifically, the sheet-shaped resin is sent out by the rotational driving of the rollers through a line contact between the sheet surface of the sheet-shaped resin in the molten state and outer peripheral surfaces of the pair of rollers. At this time, the sheet surface of the sheet-shaped resin in the molten state is cooled down during a period of this line contact. This becomes an obstructive factor against the forming by the mold in the secondary molding. Even though, increasing the rotation speed of the rollers to shorten the period of the line contact generates the neck-in like the first one, or depending on the pressing force of the rollers to the sheet-shaped resin, an excessive slippage is generated between the rollers and the sheet-shaped resin. In view of this, a failure possibly occurs in a smooth sending out of the sheet-shaped resin.
In this respect, Japanese Unexamined Patent Application Publication No. H11-005248A discloses a technique that draws the lowest portion of the sheet-shaped resin in the molten state while the sheet-shaped resin in the molten state is sent out by the pair of rollers.
More specifically, the following points are disclosed. One sheet is extruded from each of at least two dies coupled to the extrusion molding machine. Immediately after this, each of the sheets is sandwiched by a pair of sandwiching rollers. At least a surface of each of the sheets is heated, creases are smoothed out, and the surface is made glossy. These at least two sheets that are made glossy are drawn and supplied within a blow molding mold. The blow molding mold is closed and the two sheets are bonded together, and then the blow molding is performed.
More specifically, controlling the drawing speed of the sheet or adjusting the number of rotations of a screw of the extrusion molding machine corresponding to the draw-down in each of the sheets approximately simultaneously supplies both the sheets, which are made glossy, within the molding mold.
Japanese Unexamined Patent Application Publication No. H11-005248A discloses controlling the drawing speed of the sheet or adjusting the number of rotations of the screw of the extrusion molding machine corresponding to the draw-down in each of the sheets. However, this does not restrain or solve the generation of the draw-down of each of the sheets. This is merely supplying both the sheets approximately simultaneously within the molding mold by controlling the drawing speed of the sheet or adjusting the number of rotations of the screw of the extrusion molding machine based on an assumption that the draw-down is generated in both the sheets.
As described above, in Japanese Patent No. 4902789, the thickness adjustment of the sheet-shaped resin in the molten state is performed exclusively by the pair of rollers. In contrast to this, in Japanese Unexamined Patent Application Publication No. H11-005248A, the pair of rollers and the clamp are used in combination to perform the sending out of the sheet-shaped resin in the molten state by the pair of rollers and the drawing of the sheet-shaped resin in the molten state by the clamp. However, the pair of rollers make the sheet surface mirrored or glossy instead of adjusting the thickness of the sheet-shaped resin in the molten state. On the other hand, in the drawing of the sheet, both the sheets, which are made glossy, are merely supplied approximately simultaneously within the molding mold. Both are not involved in the thickness adjustment of the sheet-shaped resin in the molten state.
In this respect, as a resin used for the sheet, employing one that has a relatively large MFR value or melt tension value can prevent such draw-down or neck-in to some extent. However, this leads to a limitation of materials that can be employed, thus not being practical. Especially, when a thin walled sheet is molded, there is a case where the restriction of the MFR value cannot cope because the MFR value is preferred to be large.